1. Field of the Invention
The present invention relates to apparatuses and methods of image processing and programs therefor and, more particularly, to an image processing apparatus and method capable of appropriately imaging a subject with a high-brightness portion and a low-brightness portion, and a program therefor.
2. Description of the Related Art
In each of video cameras and digital cameras, the more the amount of light incident on an imaging device, the more the amount of charge (hereinafter, a charge) accumulated in each pixel of the imaging device, resulting in an increase in the amount of information obtained. If a subject has a high-brightness portion, charge accumulated in respective pixels corresponding to the high-brightness portion becomes saturated.
Generally, therefore, a small aperture is selected or shutter speed is increased to determine exposure so that charge accumulated in each pixel is not saturated.
Providing that the amount of incident light is limited, however, when a subject has a low-brightness portion, exposure for pixels corresponding to the low-brightness portion is relatively reduced. Although information related to the high-brightness portion of a subject image is not lost, the amount of information related to the low-brightness portion is lowered.
In image processing after photoelectric conversion, gain-up processing can be performed to make up for the lowered amount of information concerning the low-brightness portion. However, when the amount of original information is low, the gain-up processing is performed not only on a signal component but also on a noise component, thus deteriorating the S/N ratio. When the amount of gain-up is set to a low level to maintain the S/N ratio, an obtained image segment corresponding to the low-brightness portion has no gradations, poor information, black defects, and a narrow dynamic range.
When a large aperture is set or shutter speed is reduced to ensure exposure in order to maintain gradations in a low-brightness portion of a subject, the amount of information related to the low-brightness portion can be ensured in an image. Although exposure for a high-brightness portion in the image is relatively increased, charge accumulated in respective pixels corresponding to the high-brightness portion becomes saturated.
If the charge becomes saturated, an image segment corresponding to the high-brightness portion has no gradations, poor information, white defects, and a narrow dynamic range.
Various methods for ensuring a wide dynamic range are proposed.
Japanese Unexamined Patent Application Publication No. 6-141229 discloses a method (hereinafter, a first method) of combining an image signal of a high-brightness portion of a subject obtained by short-term exposure with an image signal of a low-brightness portion of the subject obtained by long-term exposure to reduce white defects in an image segment corresponding to the high-brightness portion and black defects in an image segment corresponding to the low-brightness portion.
Another method (hereinafter, a second method) is disclosed in “Fundamental Study of Wide Dynamic Range Imaging by Using Selectable-Capacitance Variable-Sensitivity Image Sensor”, Journal of the Institute of Image Information and Television Engineers, Vol. 59, No. 1, 2005. According to the second method, a variable-sensitivity image sensor has a variable-sensitivity pixel structure including two kinds of capacitances, i.e., a parasitic capacitance and an additional capacitance. When a subject has low brightness, one capacitance is used. When a subject has high brightness, two capacitances are used. Thus, charge saturation during high sensitive imaging is prevented. In this method, the sensitivity can be selected for each pixel.